The present invention relates to an optical head using a semiconductor laser (to be referred to as an LD hereinafter) as a light source and, more particularly, to an optical head using an LD array in which a plurality of LDs are linearly arrayed as a light source.
Conventionally, many proposals have been made about a method of optically recording/reproducing data on/from a data recording medium using an optical head having an LD array as a light source. For example, Japanese Patent Publication Nos. 57-60697, 58-56164, 59-9976, 59-18772, and 61-20057 are known.
FIG. 1 shows a structure of an optical head shown in an embodiment of Japanese Patent Publication No. 61-20057. A description will be made along the propagation of a light beam. Divergent light beams emitted from an LD array 102 having a plurality of light-emitting points 101.sub.1, 101.sub.2, and 101.sub.3 are collimated by a collimator lens 103. These light beams pass through polarizing beam splitter 104 and a .lambda./4 (quarter wavelength plate 105, and then form small spots 108.sub.1, 108.sub.2, and 108.sub.3 on a recording surface 109 of a recording medium 107 by an objective lens 106. The light beams reflected by the recording surface 109 pass through the objective lens 106 and the .lambda./4 plate 105. Since the light beams reciprocate through the .lambda./4 plate 105, they are reflected by the polarizing beam splitter 104, and then are converged on a light-receiving element 111 by a focusing lens 110.
Various applications of functions and roles of the plurality of small spots 108 on the recording surface 109 can be proposed. For example, assume that the recording medium 107 is moved in a direction of an arrow A in FIG. 1 relative to an optical head. The small spot 108.sub.2 is used for recording data, and the small spot 108.sub.3 is used for auto focus control and auto tracking control and for reproducing recorded data. The small spot 108.sub.1 is used for preheating a recording region or for detecting a defect or dust on the surface of the recording medium 107. These functions are realized by arranging light-receiving portions corresponding in number to the small spots 108 on the light-receiving element 111 and detecting a variation in light amount of each light beam. In the above description, the function of each small spot 108 is merely an example, and other applications may be adopted.
However, there are the following drawbacks upon employment of an optical head using an LD array as a light source having the structure shown in FIG. 1. In general, auto focus control and auto tracking control techniques are necessary for accurately recording or reproducing data on/from a predetermined position on the recording surface 109 of the recording medium 107. For this purpose, the objective lens 106 is moved to be perpendicular to or parallel to the recording surface 109 of the recording medium 107. Therefore, the objective lens 106 is required to be lightweight and compact. In consideration of this requirement, a double-aspherical single lens having a focal length fo of about 4 to 5 mm has been developed for an objective lens. In order to increase a recording density, each small spot 108 on the recording surface 109 is required to have a spot size of about 1.5 .mu.m. For this purpose, the objective lens 106 must have a feature of a very bright lens, i.e., have a numerical aperture (NA) of about 0.5. When the objective lens 106 of such specifications is designed, it is important to correct spherical aberration, an coma, and an astigmatism. However, in the optical head shown in FIG. 1, light components on and outside the optical axis, i.e., a plurality of small spots 108.sub.1 to 108.sub.3 must be satisfactorily focused on an identical plane, i.e., the recording surface 109. Therefore, correction of curvature of field must be taken into account in addition to the aberrations.
Optical components used in an optical head are designed to have a target R.M.S. value (Root Mean Square) of 0.07.lambda. or less of a total wavefront aberration of the optical components between the light source and the recording surface according the Strehl definition (.lambda. is the wavelength of an LD).
In the above specifications, a field angle of the objective lens 106 which can satisfy the above definition is about 0.5.degree.. Therefore, if the focal length fo of the objective lens 106 is 4.5 mm, an interval l between adjacent small spots 108.sub.1 and 108.sub.2 on the recording surface has an upper limit of about 40 .mu.m.
The interval l is preferably as large as possible as long as focusing performance is not impaired. The reason for this is as follows. For example, as described above, when auto focus control and auto tracking control are performed using the small spot 108.sub.3, if there is a considerably large amount of dust on the surface of the recording medium 107, it is impossible to perform accurate control using the spot 108.sub.3. In an extreme case, an erroneous recording operation may be performed by the spot 108.sub.2 at a position other than desired. In order to prevent such an erroneous operation, a change in light amount of the spot 108.sub.1 is detected to detect the presence of dust or the like in advance. An auto focus mechanism and an auto tracking mechanism (neither are shown) are fixed in position for a predetermined period of time, and are restarted later. Therefore, a given processing time is required after dust or the like is detected by the spot 108.sub.1 until the fixing operation of the mechanisms is started. For this reason, the interval l is preferably at large as possible.
Points to be noted about an interval p between the adjacent light-emitting points 101 in the LD array 102 will now be pointed out. As a form of the LD array 102, a hybrid type in which a plurality of independent LDs are linearly arrayed and fixed in a single package and a monolithic type in which a plurality of light-emitting points 101 capable of being independently driven are formed by an identical semiconductor substrate are known. In the former type, the interval p between a plurality of light-emitting points 101 is preferably large in view of an array packaging technique. In the latter type, a problem of thermal crosstalk, i.e., a problem that a signal for driving the light-emitting point 101.sub.1 leaks to influence the adjacent light-emitting point 101.sub.2, is posed. In order to avoid this problem, the interval l between adjacent light-emitting points is set to be at least 100 .mu.m or more, and is preferably set to be 150 .mu.m or more.
To summarize the above preferred requirements, a focal length fc of a preferred collimator lens 103 is determined. That is, EQU fc=p.multidot.fo/l (1)
fo=4.5 mm, l=40 .mu.m, and p=150 .mu.m are substituted in this equation to yield fc=16.875 mm.
However, when the collimator lens 103 having such a long focal length is used, the dimensions of the entire optical head are increased, and its weight becomes heavy. Therefore, such a structure disturbs a high-speed access operation of the entire optical head.
In the arrangement of the optical head shown in FIG. 1, a near-field shape of the light-emitting point 101 is directly projected onto the recording surface 109, and the small spot 108 has an elliptical or oval shape, and is not preferable for a requirement of high recording density.
FIG. 2 is a block diagram of an optical recording/reproducing apparatus described in an embodiment of Japanese Patent Laid-Open (Kokai) No. 61-206925. The apparatus shown in FIG. 2 is constituted by a semiconductor laser array 201, a collimator lens 202, an expanding prism 203, a polarization prism 204, a .lambda./4 plate 205, a converging lens 206, a disk 207, lenses 208 and 209 for converging a beam from the polarization prism 204, and a photosensor 210.
However, Japanese Patent Laid-Open (Kokai) No. 61-206925 merely describes that the semiconductor laser array and the expanding prism are used, and has no description about a detailed arrangement condition of the semiconductor laser array and the prisms for shaping a beam.